Angelman Syndrome (AS), first described in 1965, is a genetic syndrome characterized by severe mental retardation, seizure disorder, ataxic gait, lack of speech, easily provoked smiling and laughter, and sleep disorder. AS can be caused by several different mechanisms that involve chromosome 15: 1) maternal deletions of 15q11-q13; 2) paternal uniparental disomy of chromosome 15; and 3) "imprinting defects" involving chromosome 15 that cause the chromosome inherited from the mother to show functional characteristics and methylation pattern typical of a paternal chromomsome 15. Approximately 25% of AS patients do not fall into any of the above categories, and among this group there are many familial cases of AS. Many of the AS patients in this group have point mutations in a gene from 15q11-q13, UBE3A. This gene encodes a ubiquitin-protein ligase, which acts in targeting proteins for degradation by the proteasome. Mutations in this gene have the unusual property that, when transmitted from mother to child, the mutations cause AS, but when transmitted from father to child, the mutations have no phenotypic effect. Specific Aims: 1) To determine the frequency and type of UBE3A mutations in AS patients who show normal results of fluorescence in situ hybridization and methylation analysis. 2) To determine the frequency with which UBE3A mutations in AS patients are de novo versus inherited from a phenotypically normal carrier mother.